Some more science considerations/thoughts ….

P. Coppi, Yale

?

?

E>5 GeV?

E>30 GeV?

vs.

Don’t forget absorption byinfrared/opticalbackground!

Real population statistics and fully observed SED peaks would be very useful …

Numerical simulations for 3C 279. Numerical simulations for 3C 279. Spada et al. 2001Spada et al. 2001

In case you still thought things were simple…

Mkn 421 2002 X-ray/TeV campaign

(Dieter Horns, preliminary)

X-ray

TeV

X-ray hardness ratio (spectrum)

Counts

If t_var = 6 hours (one night) - one telescope won’t do it!!

Lesson from ASCA/X-ray monitoring days…. Need complete time sampling!

Typical HESS/Veritas observation?

VHE (GeV-TeV) gamma-ray emission is a highly timevariable phenomenon.

We need a “Gamma-Ray Timing Explorer” (GTE) analog to the Rossi “X-Ray Timing Explorer” (RXTE) with the same relative sensitivity at ~ 1 GeV as RXTE at ~1 keV – with no coverage gaps …

…. Ideally, while GLAST is up!

(HAWC won’t do this. Would be nice to have similarthreshold to GLAST so see same sources. )

GLAST and GRBsLong burst w/optical flash detected by ROTSE, BATSE flux > 99.6% BATSE bursts

Briggs et al. 1999

Energy Flux at MeV Peak6 -2 -14 10 erg cm s

Integration Time for Spectrum ~ 32 s

Assume same energy flux at 1 GeV,4 210 cm collection area,

~ 800 photons

Great GeV energy spectrum forthis burst, and reasonable spectrafor bursts ~ 50x fainter.A MAJOR improvement over EGRET!

BUT … this is a time integrated spectrum…Look at what BATSE saw during those 32 sec

GLAST and GRBsAwesome statistics, even for64 msec time bins.

Allows detection of significantspectral variability on < 1 sectimescales.

Just as for blazars, fitting time-integrated spectra when thissort of variability is going on isNOT a good idea.

Can GLAST match this X-ray sensitivity?

GLAST and GRBsAssume constant GeV flux at peak count rate (optimistic!):

N_photon in 1 sec @ 1 GeV = 25 -- o.k. N_photon in 64 msec @ 1 GeV = 1.6 -- not too useful

Also, although GLAST has sensitivity at 10 GeV, N_photon in 1 sec @ > 10 GeV ~ 2.5 -- not too useful

GLAST is marginal, and this is for a very bright burst!(N.B. OSSE detected 16 msec variability for this burst at ~ 1 MeV.)

GLAST and GRBs

1 -7 -2 -1Assume standard decay rate, and GeV flux at 100 sec of 4 10 erg cm s

(same as X-ray flux), then N_photon in 180 at 1 GeV (good), and

~18 at 10 GeV.

O.K. for spectrum up to 10 GeV. Good news

t

t t

! ... But might be very optimistic

because emission peak could move quickly through GeV observing window.

Another key component of GRB studies is the AFTERGLOW. Can GLAST study this?

[Afterglow is much easier because there is no rapid time variability.]

Bottom line: Unless we’re lucky with physics, GLAST will only seebrightest bursts at ~ 1 GeV, and there is not much margin for error.

M87 – FRI (weak jet)

X-RAY

Mostly synchrotron emission?

Hey, there are some interesting nearby objects – jet emission (synch X-ray? => TeV e-/e+)!

Resolved X-ray emission -> in situ acceleration!?

D. Harris,2003

M87 jet is not wimpy!!!

X-ray variability seen in HST-1 knot too!!

An accurate measurement (upper limits) on the GeV-TeV extragalactic diffuse background.

Why so interesting?

GeV-TeV+ gamma-rays only produced in extreme environments or by “exotic” processes: e.g., black hole jets, supernova blast waves, cosmic strings, relict particle decays, or matter-antimatter annihilation.

Background is sum of all nearby GeV-TeV activity in the Universe + all > GeV activity at z > 1.

[ Gamma-ray pair production and cascading on intergalactic photon fields

GLAST = calorimeter for VHE-EHE Universe!

(best limits on BAU/matter-antimatter domains from gamma-rays) ]

Blazar Background Models, a la Stecker & Salamon 1996

Including IR/O absorption

Don’tforgetcascades!

Coppi & Aharonian 1997

[~MeV]

Klein-Nishinaeffects important?

Be careful in interpreting originof spectral featuressuch as “bumps” and break energies!

Can get spectral indexharder than 0.5!

ERC,blackbodytargets

ERC,power-lawphotontargets

Moderski et al. 2005

EGRETblazars?

Some TeVblazars?

[N.B.: Getting strongTeV emission not so easy!]

Fun stuff: clusters ….

Expected flux levels extremelyuncertain!

Most sources can think of, even decaying/annihilating CDM particles, trace large scale structure/shocks… look for clustering signal!

Bromm et al. 2003

Low threshold science objectives:

GLAST AGN follow-up

UV/optical EBL

Diffuse gamma-ray background (extragalactic and galactic)

GLAST “hotspot” follow-up

GRB, high energy components

Microquasars (NIR jet emission detected)

SNR/Cosmic Ray accelerators

Pulsed emission from plerions (pulsars )

Galaxy clusters

UHECR sources/”Haloes”

Star formation-related cosmic ray emission from other galaxies

??? Serendipity: Exciting particle physics?

What if your “low energy” threshold is 30 GeV?

Don’t go halfway or risk losing GLAST-related science!And do a bad of “TeV” science…

Aside: really pounding away at >1 TeV relatively easy and interesting too…(cosmic ray, SNR, probe EBL in 10-60 micron region – most poorly constrained by direct counts & impacts star formation history

Theorist’s Wish List

Rule of thumb: give a theorist a spectrum consistent with a power law(e.g., due to insufficient statistics) and he can fit any model/EBL you like.

Need to detect curvature! Ideally measure both sides oflow and high energy peaks, simultaneously w/good(< hour-month) time-sampling: UV-MeV, 100 MeV-TeV coverage. [Also very good to get below IR/O absorptionthreshold.]

There will always be some special objects,e.g., Mkn 501, not accessible from a givenground-based site...

Want good population statistics ….

One “super” telescope not enough – want tightly coordinatedspace and ground-based telescopes.

As gamma-rays enter realm of mainstream astronomy, similar considerationsfor future progress apply as for other sub-fields of astronomy:

a) Large area survey capability

b) Improved Sensitivity

c) Angular resolution!!! (big problem at GeV?)

d) All-sky monitoring for variable sources (what will replace GLAST? Mostblazars seem to be dead most of the time…)

e) No gaps in time coverage/high duty cycle…

f) As broadband/multiwavelength observations as possible!(Think about connections to other instruments/missions, e.g., hard X-ray telescopes like EXIST.)

Given current technology, no single instrument configuration or oneInstrument can do everything….